The nitration of toluene can produce three MNT isomeric products. Most industrially-produced MNT is further nitrated to dinitrotoluene (DNT), which is used in the production of toluene diisocyanate (TDI), a component of polyurethane. However, some MNT production focuses on the pure MNT isomers for smaller specialty chemical markets with the major uses being the production of dyes, rubber chemicals, and agricultural chemicals.
The production of MNT, either as a product in itself or as an intermediate for DNT, is typically carried out using isothermal mixed acid nitration. The “mixed acid” or “nitrating acid” is a mixture of nitric and sulfuric acids with sufficiently strong acidity to generate the nitronium ion (equation 1), which is the key reactant for the nitration reaction (equation 2).H2SO4+HNO3H2O+NO2++HSO4−  [1]NO2++HSO4−+C6H5CH3C6H4CH3NO2+H2SO4  [2]
The toluene has a low solubility in the mixed acid and so forms a second phase when they are mixed together. Because of this low solubility of toluene in the mixed acid and the rapid rate of the nitration reaction, the reaction takes place in the acid phase within a thin diffusion layer. The reaction is also exothermic. Because of these two factors, industrial reaction vessels are well mixed both to generate a large interfacial area between the two phases for the reaction and to provide good heat transfer to the reactor cooling coils to keep the reaction isothermal and avoid thermal runaway. To avoid excessive by-product formation, the reactions are typically carried out at around 25-60 degrees C.
The overall result of equations 1 and 2 is the generation of MNT and water, with the sulfuric acid acting as a catalyst and not being consumed. The product from the reactor is typically decanted to produce an organic product phase and a “spent acid” phase. The spent acid phase contains the sulfuric acid catalyst, but also the water generated by the overall nitration reaction. Because of this, the acid must be re-concentrated before it can be re-used. This involves heating the spent acid, typically under some vacuum, to remove the excess water, and this represents a significant energy demand for this process, as shown in the isothermal nitration of FIG. 1.
Adiabatic nitration involves carrying out a carefully controlled amount of nitration without reactor cooling, resulting in a targeted amount of temperature rise of the organic and nitrating acid mixture. After product separation, the hot spent acid is re-concentrated by flashing under vacuum. In this way the reaction heat is used to drive the spent acid re-concentration, resulting in a significant energy savings. This approach is widely used in the production of mononitrobenzene (MNB) from benzene and it would be highly desirable to use adiabatic nitration for the production of MNT. However, to achieve the maximum energy savings, this acid recycling loop requires integration of the nitrator temperature rise with the acid re-concentration step. A further constraint is imposed by the economically available vacuum for the acid re-concentration, which then requires a sufficient temperature to achieve a given concentration of sulfuric acid. For the transition of MNB from isothermal to adiabatic production this resulted in increased nitrator operating temperatures, which would be expected to lead to higher by-product formation. However, the increased reaction rate at the higher temperatures allowed for shorter reaction times, and the ease of acid re-concentration made operating with less total reaction per pass practical, with the net result being good product quality and significant energy savings. However, toluene is more susceptible to oxidation side reactions than benzene, and also the product MNT is more easily oxidized than MNB, and so shifting to higher reaction temperatures as would be required for an adiabatic MNT process is more problematic due to the increased potential for by-products. The integration of the nitration reactor and the acid re-concentration also decreases the temperature of the acid re-concentration, which has the potential to cause problems with the build-up of impurities in the acid loop that might otherwise be decomposed or vaporized at a higher acid re-concentration temperature (see for example Demuth et al., U.S. Pat. No. 6,583,327).
There are three routes to by-products during the nitration of toluene to form MNT. The largest amount and range of by-products is formed by nitric acid-driven oxidation reactions of both the toluene and the MNT product. These reactions lead to a range of products including cresols (methylphenols) and benzoic acids as well as products from their further oxidation and nitration. The second class of by-products is generated by over nitration and predominately consists of dinitrotoluenes (DNTs). The potential third class of by-products is sulfonated compounds generated by reaction of aromatic compounds with strong sulfuric acid, though these tend to be significant only at higher sulfuric acid concentrations (more than about 75 wt % sulfuric acid).
Industrially, the oxidation by-products are removed by alkaline washing, generating a highly colored aqueous waste stream referred to as “red water.” Oxidation by-products therefore represent a waste of chemicals as well as additional costs related to the disposal of the red water. In DNT production, it is reported that the isothermal toluene to MNT stage can generate on average about 0.72 wt % cresols. Thus any adiabatic process should not generate significantly more than this amount.
The key difference in requirements between MNT production for MNT itself versus MNT as an intermediate for DNT, involves the acceptable levels of DNT by-product. If the product MNT is to be further nitrated to DNT, then the presence of by-product DNT is obviously not of concern. However, if the target product is MNT, the DNT by-products must be removed. The DNT by-products remain in the product after the alkaline washing and are typically removed as part of the distillation of the product MNT into its isomer fractions. As well as representing a waste of chemicals, high levels of DNT are a processing concern for the distillation. This is because DNT is less thermally stable than the MNTs and so care must be taken in the operation of the distillation process, with the related dangers and operational issues becoming more severe with higher DNT levels in the MNT product.
Therefore, for an adiabatic MNT process to be successful it should not unduly increase the by-product levels versus the isothermal process. Within this patent application, targets for the sum of cresols and benzoic acids less than 1 wt % (<10,000 ppm) and for DNTs less than 0.5 wt % (<5,000 ppm) are considered as consistent with a viable adiabatic process for the production of MNT.
An adiabatic approach to the production of MNT has been described in Konig et al., U.S. Pat. No. 5,648,565. However, the tests described in Konig et al. are based on simple batch testing using a stirred beaker. It is known that operating adiabatically using conditions as specified in Konig et al. but including re-concentrating and re-using the sulfuric acid quickly results in a build-up of organic compounds in the recycled acid. See, for example Demuth et al., U.S. Pat. No. 6,583,327. This would pose a problem for long-term industrial operation of the process described in Konig et al.